Okay, let's do the math on a six pack; center carb is a 350 cfm, the two end carbs flow 500 cfm each. I hope I don't need to add this up for you. Go on youtube and go to Nick's Garage and watch a 440 six pack on his Super Flow dyno. He has a camera looking down the carbs, now tell me they aren't completely open at WOT.

Those numbers are the two barrel ratings. It isn't the same as a 4 bbl rating. It was a marketing gimmick to fool people into thinking they had 1350 cfm when they actually had a lot less.

OP has a very mild 440 engine. It isn't going to pick up much power on the dyno with a bigger carb. It will only use about 550 cfm at WOT so a bigger carb isn't going to add much power. Might pick up 5 hp with a 750 carb. It would still drive okay with a 750 carb, especially if it has vac secondaries. I doubt it would pick up power with an 850 over a 750. Very likely that it would start to be a bit soft off throttle with a 850 Holley, depends a bit on the gear ratio out back. A Thermoquad or QJet could be made to work on that engine but the secondaries would most likely never open all the way.

LOL..these threads are always fun. I have a 69 B'cuda 4 spd 3:73 gear street toy. 1st motor was a thrown together 383 with a purple cam. Started with a hand me down 650 Holley DP...put gaskets in it and off we went. Ran excellent! A friend got me a deal on a 3310 750.....never touched it...bolted it on....ran excellent! Built a 470" w/solid roller. Left the 3310/750 on it....guess what....ran great! Got my hands on a 800 DP. Went through it ...gaskets etc. Bolted it on the 470...guess what....runs great!

Moral of the story? Many options out there for street toys! Will they be optimum options...likely not. Will they hit "the number" on an a/f sensor...likely not. Will the OP be able to start/run/drive/enjoy his toy?...most likely YES!!

PS..forgot...My vote is a 3310 Holley. Great all around street carb.

Cubic Feet Per Minute (CFM) is the same whether it's a 2bbl or 4bbl, that's what the carb flows. It makes no difference if it's a 2bbl or 4bbl, the CFM is what that carb delivers. There's no difference in the 2bbl or 4bbl rating.

not buying your comment on hp related cfm requirements. The size of the engine/rpm has a lot to do with it.

Your better off looking at venturi area as a better comparison.

And even if I cant add, I can see there is more holes of similar size than a 750.

Obviously you have never ran an O2 sensor to see how rich those 3310s are at cruise which A)wastes gas B)washes down the cylinder walls C)leaves extra combustion chamber deposits D)dilutes and blackens the oil E)stinks up the place.

Our engines are much more tolerant of overly rich conditions than they are of lean ones. And overly lean conditions can damage our engines much faster and more dramatically than rich ones. That is why carbs that are overly rich are so often touted as a "Great all around street carb". Their very real down sides are not nearly as obvious.

The 3310 can be made into a pretty good street carb fairly easily and is worthy of consideration. But making it in into a truly "Great all around street carb" is another matter entirely.

Just because the carb doesn't exhibit any nasty driving habits by being too rich does not mean that it is not slowly doing real damage to the engine. A carb does not have to be perfect to be acceptable, but there are more things to consider other than it drives okay. A cheap narrow band will help out lot with these 'universal' carbs. Well under $100 will set one up. For a street car, the cruise is the place where the car spends a lot of time typically and is the area that should be addressed. A properly sized carb will be cruising on the main circuit which is nearly always easily adjustable. A narrow band can get you into an acceptable cruise ratio without too much hassle.

The 2bbl carbs are measured at a different inlet depression than a 4bbl carb. They also have changed wet vs. dry flow over the years.

https://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

Power comes from fuel so the amount of power produced is directly related to the amount of fuel consumed. 400 hp requires roughly 200 lbs of fuel per hour. The air fuel ratio in a street engine at WOT is around 13:1 so that gives you how many pounds of air you need. Once you know pounds of air you can convert to CFM. Engine size and RPM aren't required in the equation since the power comes from the fuel consumed.

Nope

https://www.speed-talk.com/forum/viewtopic.php?t=32644

https://thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

the 6bbl would be rated at about 810 cfm if flowed the same as the 4bbl. Better distribution is what made it shine.